Recently, I've connected two devices using a 20 meter long Ethernet cable. This solved a huge problem with my Wi-Fi (as I've reduced the distance from 20+ to 4 meters and also avoided some obstacles to the signal), but another problem emerged. My connection to the Internet looks like

computer --(Wi-Fi)-- device1 --(ethernet)-- device2 ....

My connection to device1 works always well (<1% packets lost), but my connection to device2 makes strange things: sometimes it works well too, sometimes 50% packets get lost, sometimes nearly all of them. The behavior changes very rapidly, two measurements in row may give totally different results, as if there were some strong jamming. The result from running

Resetting of the devices doesn't really help (it does in case it malfunctioned completely, which happens from time to time). AFAIK, the maximum Ethernet cable length is 100 meter, so this should be no problem. Moreover, I'd expect some signal deterioration leading to consistently lowered transmission rate or maybe a complete failure, but not this.